You are not "adding" the MD state pickup.  The add is on the W-2.  The subtract is on the 1099-R.  As long as there is an amount and not an EP, then you put the letter P in the State Use Code box and the amount in Box 19-1 Local Distribution.  Verify on the MD return from the subtraction worksheet and if a carryover for unused amounts, verify that too on the same 502SU.